The long-term of this proposal is to obtain a better understanding of the functional role of steroidogenesis in the developing rodent yolk sac placenta. To accomplish this goal, we propose to investigate the nature of the pathways for steroid metabolism, the developmental pattern of steroid-metabolizing enzymes and the factors which regulate steroidogenesis in both the parietal and visceral portions of the yolk sac placenta. Furthermore, we propose where possible to compare the results of these studies with similar studies carried out on the rodent chorioallantoic placenta and ovary. Pregnant rats at various stages of gestation will be used in these studies. At sacrifice, the parietal and visceral yolk sac, chorioallantoic placenta and ovaries will be isolated and tissue homogenates will be prepared. The homogenates will be assayed for a variety of steroid-metabolizing enzymes, including: 3 beta-hydroxysteroid dehydrogease, delta 5 yields delta 4 isomerase, 5 alpha -reductase, 17 beta-hydroxysteroid dehydrogenase, 17 alpha -hydroxylase, 17-20 lyase, 3 alpha -hydroxysteroid dehydroxysteroid dehydrogenase, 20 alpha -hydroxysteroid dehydrogenase and aromatase. The assays will becarried out by incubating labeled steroid substrates (e.g. 3H-pregnenolone, 3H-dehydroepiandrosterone, 3H-androstenedione, 3H-testosterone, 3H-progesterone, 3H-17-alpha -hydroxyprogesterone, 3H-cholesterol, 3H-estradiol) in the presence of appropriate co-factors (NAD+, NADPH, etc.) and then isolating newly-synthesized radioactive steroid products by thin layer chromatography and finally determining radioactivity by liquid scintillation spectrometry. Aliquots of the tissue homogenates will be used to determine the content of progesterone by the method of radio-immunoassay. Using the methods described, the uptake, biosynthesis and metabolism of steroid hormones also will be studied in early postimplantation whole-rat embryo explants grown in culture during the period of organogenesis.